Bovine virus diarrhoea virus (BVDV) is extremely common in cattle in the UK, the remainder of Western Europe, North America, Australia and Africa. Infection with this virus may result in a variety of syndromes and pathologies influenced largely by the age of animals when first infected. In young, previously uninfected calves the virus causes a transient infection. This is associated with leucopenia, and an interrelated period of immunosuppressive and increased susceptibility to infection with other microorganisms. BVDV is, after RSV (respiratory syncitial virus), probably the most important virus associated with outbreaks of respiratory disease in young housed calves and because of its immuno-suppressive effect it may be involved in other calf infections, for example enteritis. This virus is also considered to be a major contributor to disease in xe2x80x9cfeedlot calvesxe2x80x9d in the USA and Canada. Following recovery, animals exhibit a degree of immunity to reinfection. However, this immunity appears not to be absolute or lifelong.
More serious problems result from infection of pregnant cattle. Abortion may ensure or alternatively deformities may be produced in the foetus that is carried to term; these deformities may result from exposure to virus at the time when immunocompetence is developing and could be the result of an incomplete immune response. Infection of the feotus before immunocompetence develops can result in the foetus remaining viraemic through the period of gestation and the subsequent birth of a calf that remains persistently viraemic, with a non-cytopathogenic form of the virus, and specifically immunotolerant to BVDV for life. Such calves are the animals that die later of mucosal disease; an event triggered by superinfection with a cytopathogenic variant of BVDV.
It has been estimated that about 0.4% of apparently normal beef calves in the UK are viraemic and these animals represent a major source of infection on farms.
Traditionally, viral vaccines fall into two classes: live vaccines containing live viruses which have been treated or grown (attenuated) in such a way as to make them less pathogenic, and vaccines containing killed (inactivated) virus particles. In the context of BVDV, the viruses themselves may be cytopathogenic or non-cytopathogenic. Thus, in principle, four main classes of BVDV vaccine could exist, although the vast majority of commercial vaccines are based on the cytopathogenic virus. Moreover, it is thought by many that live vaccines are unacceptable because live cytopathogenic vaccine strains may produce death from mucosal disease in persistently viraemic animals, and live non-cytopathogenic virus vaccine may infect the foetus in pregnant cattle and result in any of the diseases outlined above.
Infection via the respiratory tract is probably the most important route of transmission of the virus on farms and protection against spread via this route would be expected to have a major beneficial effect in controlling disease due to BVDV.
Parenteral vaccination with inactivated BVDV protected against respiratory infection. In one experiment all of 5 vaccinated calves were resistant to respiratory challenge and all of 5 controls become infected.
The killed BVDV antigens tested induced the production of high titres of neutralising antibodies. These were shown to rise from less than 50 before vaccination to greater than or equal to 2,000-10,000 units after vaccination.